Gravimetric Analysis of Phosphorus in Plant Food

Gravimetric Analysis of Phosphorus in Plant Food

This experiment uses a technique known as gravimetric analysis to determine how much phosphorous (as a weight % P2O5) there is in plant food.

Safety Concerns: This lab uses plant food, MgSO4, and ammonia. The principal hazard in this experiment is contact with irritating solution and vapor. The chief hazard is aqueous ammonia which will burn skin and irritate (and potentially harm) the eyes. Do not smell the ammonia and be careful not to spill any on the skin. If any of the reagents in this lab are spilled on the skin, wash copiously with water. If ammonia vapor irritates the eyes, either use the eye-wash to rinse the eyes out and/or step out of the lab until relieved.

As much as possible, use the ammonia in the hood.

All solutions used in this lab are low hazard household chemicals. Dispose of them in the appropriate container in the lab.

Background:

Plant foods and fertilizers are commonly characterized by three numbers: 1) weight percent Nitrogen; 2) weight percent phosphorus (as P2O5); 3) weight percent potassium (as K2O). In this experiment, we will check the number corresponding to the “phosphorus” content.

P2O5 is the acid anhydride of H3PO4 (phosphoric acid, the acid that has phosphate ion, PO43-, as its anion). An acid anhydride is derived from dehydration of the corresponding acid. Therefore, mixing an anhydride with water will give the corresponding acid. The equilibrium that exists is shown in Equation 1. As shown, the reaction is unbalanced. Take time now to balance the equation.

Consequently, when a plant food is dissolved in water, the phosphorous-containing species is converted to something more like H3PO4 than P2O5. The reason we don’t say that the species is H3PO4, is that in solutions that have a range of acidities, the actual species in solution could be H3PO4, H2PO4—, HPO42—, or PO43—, with more acidic solutions favoring the protonated (i.e.,...